The present invention relates to a method of producing a printed circuit board having through-holes.
In a conventional method of forming a conducting path between the two sides of a printed circuit board having circuit patterns formed on one or both sides of an insulating substrate, as shown in FIG. 2, such work has been carried out by penetrating a through-hole 4 between lands 2 and 3 of the circuit patterns formed on the insulating substrate 1 and then packing conductive paste 5 into the through-hole 4 by silk-screen printing or the like.
When forming the circuit patterns with a copper-film-cladded laminated board, the through-hole 4 is usually bored by punching or drilling prior to forming the circuit patterns, and then solder resist 6 is adhered after forming the circuit patterns by etching.
When packing conductive paste 5 into the through-hole 4 in the manufacture of a conventional printed circuit board, it is also necessary to build up conductive paste 5 on the lands 2 and 3 to secure an electrical connection.
However, while the paste 5 is usually packed by using a printing means such as silk-screen printing, this technique has a drawback in that the paste spreads or sticks on the outer periphery of the narrow lands 2 and 3. This has been a serious problem, because it often generates defects owing to bridge formation between adjoining lands and may also deteriorate the electrical insulation between lands, as high-density printed circuit patterns become practical.